An electrostatic precipitator generally comprises a housing structure provided with a gas inlet and a gas outlet, a multiplicity of dust-collecting plates suspended in this housing for collecting particles of dust from the gas on these plates, means for dislodging the dust from the plates and for collecting the dislodged particles, and corona discharge electrodes which charge the dust particles to a polarity opposite that which is applied by a high-voltage source across the electrodes to the collecting plates.
Electrostatic precipitators may have sealing covers or roof structures, i.e. so called ceilings which can be composed of sheet metal (see inter alia commonly assigned U.S. Pat. No. 4,248,610 and the references therein cited).
Such sheet metal members can rest above support beams extending horizontally across the top of the housing and from which the collecting plates may be suspended. Metal strips can overlie the seams or edges of these metal sheets and can be welded to them to form a gas tight seal whereby the roof structure can be described as a sealing roof.
In electrostatic precipitators operated by a subatmospheric pressure of 10 to 50 mbars below atmospheric pressure, sealing roof structures of this type are generally satisfactory because of their structural simplicity, ease of installation and economy. However, they are not capable of withstanding even brief pressure rises which have come to be expected in the operation of such electrostatic precipitators with combustible gases.
The increasing concern for environmental protection has lead to greater use of electrostatic precipitators to recover dust particles from gases which may contain combustible components. Such gases can be standard flue gases from combustion chambers, e.g. boiler combustion chambers, metallurgical plants, chemical plants or the like, and even for product gases or byproduct gases, e.g. of metallurgical plants, which can contain combustible components.
During the operation of an electrostatic precipitator or electrofilter, it is not uncommon that a number of detonations are generated by ignition of such combustibles when concentrations thereof reach the explosive limit. Such detonations produce pressure waves which, at the sealing roof, may be manifested as pressure surges of up to 100 mbars. In the past such pressure surges have lifted off the sheet metal elements, broken the seals, and damaged the sheet metal elements. Indeed, experience with conventional constructions have shown that even at pressures of 5 mbar above atmospheric, sheet metal elements of the aforedescribed type can be lifted off. This is obviously a disadvantage since each time such a pressure surge occurs, it is necessary to repair the damaged roof structure which may involve some downtime of the electrostatic precipitator.
It should be noted that the beams on which the sheet metal elements rest are so-called I beams, i.e. generally have upper and lower flanges interconnected by vertical webs and with these beams are capable of withstanding considerable force since they function as structural supports for the collecting electrodes and to provide a supporting surface for the roof sheets which can be overlapped by them. The beams generally extend in the direction of gas flow, and have the same spacing as the arrays of collecting electrodes.